The present invention relates to control of product purity in a pressure swing adsorption system; and, more particularly, to a method and apparatus for automatically controlling product purity without risking unacceptable impurity breakthrough as the feedstock changes, yet providing rapid response and high stability.
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multicomponent gas stream containing at least two gases having different adsorption characteristics. The more-strongly adsorbable gas can be an impurity which is removed from the less-strongly adsorbable gas which is taken off as product; or, the more-strongly adsorbable gas can be the desired product, which is separated from the less-strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more-strongly adsorbable gases, such as ethylene, from a feed to produce an ethylene-rich product.
In pressure swing adsorption, a multicomponent gas is typically fed to one of a plurality of adsorption beds at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, feed to the adsorber is terminated and the bed is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less-strongly adsorbed component or components remaining in the bed to be drawn off without significant concentration of the more-strongly adsorbed components. Then, the bed is depressurized by a countercurrent depressurization step wherein the pressure on the bed is further reduced by withdrawing desorbed gas countercurrently to the direction of feed. In multi-bed systems there are typically additional steps, and those noted above may be done in stages. U.S. Pat. Nos. 3,176,444 to Kiyonaga, 3,986,849 to Fuderer et al, and 3,430,418 to Wagner, among others, describe multi-bed, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in their entireties.
It is known that controlling product impurity level, e.g., in the less-strongly adsorbed component, to the maximum allowable level results in the highest system efficiency. It is also known that the primary means for controlling product impurity level is to adjust the time each adsorber spends in the adsorption step. If the product impurity level is too high, the adsorption step is shortened, and vice versa. However, when processing feedstocks of a variable nature, e.g., a feedstock comprised of several different streams which may not all be present at all times, it is difficult to control the product purity concentration without unacceptable impurity breakthrough as the feedstock changes.
In conventional systems, the operator monitors the product impurity level and manually adjusts the adsorption step time. This manual process can be automated through a feedback control system. In such a system, the product impurity level would be sensed, and a controller would adjust the adsorption step time depending on the difference between the actual and desired impurity level. Such a system, however, suffers from the usual disadvantage of feedback control; that is, corrective action can only be taken after the undesired event (too high or low impurity level in the product) has occurred.
A feedforward control system could be used alone or in conjunction with the above feedback control system. The feedforward system would be much more complex. The feed composition and flow would have to be measured on-line and the measurements would have to be input into a process model in order to determine the magnitude of the corrective action. The feedforward system has several disadvantages, including the following: (1) feedforward control systems are inherently less stable than feedback control systems; (2) a system which can accurately analyze the concentrations of the components in a multi-component system would be extremely complex and expensive; and (3) an overly-simple and inaccurate process model would have to be used due to practical process control system limitations.
There remains a present need for a method and apparatus for automatically controlling the quality of product from a pressure swing adsorption system which could maximize system efficiency not only for feeds of constant composition but also for feeds which vary in composition and/or flow rate, pressure levels, or temperature, as well as systems operating with other variable process parameters.